The present invention relates to switching regulators and driver circuits. In particular, the present invention relates to a method and apparatus that provides for an improved temperature coefficient for the current limit in a switching regulator circuit. The improved temperature coefficient in the current limit may also be employed for use in a driver circuit such as an RS-232 driver.
Current limit is an important parameter for switching regulator and driver circuits. A precisely controlled value for the current limit in a wide temperature range is always desired. An example of a current sensor circuit (100) that may be used in a switching regulator circuit is shown in FIG. 1. Switching regulator circuit 100 includes a comparator (110), a differential gain amplifier (120), a reference voltage (VREF), a bi-polar junction transistor (BJT) (Q1), a diode (D1), an internal voltage reference (VREF(I)), a sense resistor (RS), and two resistors (R1 and R2).
Comparator 110 includes a sensor input (SNS) that is coupled to node N10, a reference input (REF) that is coupled to node N11, and an output that is coupled to node N16. Differential gain amplifier 120 includes differential inputs that are coupled to node N12 and power supply node NPS10, and an output that is coupled to node N10. Internal voltage reference VREF(I) is coupled between node N11 and node N13. Diode D1 is coupled between node N13 and node N14. Transistor Q1 includes a base that is coupled to node N15, an emitter that is coupled to node N14, and a collector that is coupled to power supply node NPS11. Resistor R1 is coupled between node N15 and power supply node NPS12. Resistor R2 is coupled between node N15 and power supply node NPS11.
The resistors (R1, R2) operate as a voltage divider. In this example, sense resistor RS is located xe2x80x9coff-chipxe2x80x9d as can be resistors R1 and R2. A circuit ground potential (GND) is coupled to power supply node NPS11.
In operation, comparator 110 produces an output signal when the voltage signal level at node N10 exceeds the signal level at node N11. Differential gain amplifier 120 produces the output signal based on the voltage drop across the sense resistor (RS). The voltage drop across the sense resistor (RS) is equal to the product of the value of the sense current (IS) and the value of the sense resistor (RS). The differential gain amplifier (120) then takes the resulting voltage value (ISxc2x7RS) and scales it (e.g., xc3x973). This scaled value is outputted by differential gain amplifier 120 at node N10. The sensor input (SNS) receives a signal from the output of the differential gain amplifier (120).
Similarly, the reference input (REF) receives a signal from the output of transistor Q1. Transistor Q1 produces the signal based on the voltage present across resistor (R2) of the voltage divider (R1, R2) due to the reference voltage (VREF). Diode D1 provides an offset to compensate for the voltage drop (VBE) across the base-emitter junction. The internal reference (VREF(2)) voltage is provided as a design adjustment. Therefore, the signal received at the reference input (REF) of the comparator (110) is proportional to the reference voltage (VREF(1)). The equation for the switching regulator circuit (100) of FIG. 1, when the sensor current (IS) is at its peak value, can be expressed as follows:
3xc2x7ISxc2x7RS=[(R2xc2x7VREF(1))/(R1+R2)]xe2x88x92VREF(2)
IS={[(R2xc2x7VREF(1))/(R1+R2)]xe2x88x92VREF(2)}/(3xc2x7RS)
The value of IS depends on the ratio of R2/(R1+R2), the value of VREF(1) and VREF(2), and RS. Assuming VREF(1) and VREF(2) have no appreciable temperature coefficient, the temperature coefficient of IS is only dependent upon the temperature coefficient of RS. RS may be an equivalent resistance such as the on resistance (RDS(ON)) of a MOSFET transistor in a switching regulator. In this instance, the ISxc2x7RS has a large temperature coefficient that is on the order of 4000 ppm/xc2x0 C., which is intolerable in some applications.
The present invention is directed to a method and an apparatus that improves the temperature coefficient for the current limit in a switching regulator, and also in driver circuits. An improved switching regulator/driver circuit includes xe2x80x9con-chipxe2x80x9d resistance circuits that allow for a reduced temperature coefficient associated with the current limit. The improved temperature coefficient of the current limit may be arranged to provide for a constant current limit in the switching regulator or driver circuit. High output currents are limited over a wide range of temperature changes, providing for improved protection to the switching regulator or driver circuit.
Briefly stated, a method and apparatus is provided that is directed to generating an improved temperature coefficient for the current limit in a switching regulator circuit. A current limit sense circuit is employed that includes a comparator that compares two signals to determine when the current limit has been exceeded. One signal is produced from an input voltage source that has no temperature coefficient, a trans-conductance cell, and a sensor resistor circuit. An active output circuit produces another signal that corresponds to the current associated with the switching regulator circuit. The current sensed by the regulator is temperature dependent due to the resistances in the active output circuit, the sensor resistor circuit, and the trans-conductance cell. Each of these resistances has a temperature coefficient. The sum of the temperature coefficients determines the amount of temperature dependence in the sensed switching regulator circuit current. The resistance materials are chosen such that the temperature dependence of the sensed current is minimized. Also, all resistors are integrated into a single chip, reducing the costs associated with external pins and external resistor components. A similar arrangement may be applied to a driver circuit with a limited current output.